health: fix magicsockReceiveFuncWarnable health clearing

Fixes #13204

Change-Id: I7154cdabc9dc362dcc3221fd5a86e21f610bbff0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f